Almost everyone has seen dental mirrors and knows something about their utility. One of the main functions of a dental mirror is to allow the dentist to view the commonly unseen sides of a persons teeth, which sides face the interior of the mouth. Up to now, dental mirrors consisted of a round spectral disk mounted at an angle at the end of a long slender rod. To use the mirror the dentist, depending on whether he was right or left handed, held a working tool, such as a drill in one hand, and the mirror in the other hand. Thus both hands were occupied, and if another tool is needed, obviously, the first tool had to be set down before the second tool could be picked up. One can see that the dentist was limited to using one tool at a time.
As the art of dentistry progressed, the dentist discovered that in some cases, he could perform a better job if he could use two working tools at the same time instead of having one hand tied to a dormant device such as the mirror, which is only an aid to the dentist in performing his duties. In such situations, some dentists summon an assistant to help them. However, this is undesirable, because too many hands would be located within the region of the mouth.